1. Field of the Invention
The present invention relates to a feed device for a sweep beam array antenna. Such an antenna is intended to produce a beam in which the position of the maximum is controlled by a number of phase-shifters disposed in the feed lines.
2. Description of the Prior Art
For a number of reasons and more particularly for reasons of cost and reliability, it is desirable to reduce as much as possible the number of control phase-shifters.
Determination of the minimum of phase-shifters is known; it depends on a number of factors among which may be mentioned:
F.sub.1 (.theta.): directivity of an elementary antenna; PA0 D.sub.o : spacing between elementary antennae; PA0 .theta..sub.o : sweep range of the beam.
The total diagram may be written in the methematical form: EQU E(.theta.)=F.sub.1 (.theta.).times.F.sub.2 [Do(sin .theta..sub.1 -sin .theta.)]
in which F.sub.2 is maximum for the values of its argument equal to k, k being a positive, negative or zero integer.
Depending on Do, the spacing between two elementary antennae, there will be a principal maximum for .theta.=.theta..sub.o and secondary maxima equal to the principal for: EQU sin .theta..sub.p -sin .theta..sub.o =.+-.K/Do
but these secondary maxima are undesirable for they give false directional indications. The useful sweep range of the beam is then limited by the appearance of these secondary maxima. One means for solving this problem of limitation of the sweep range consists in striving for elementary diagrams such that F.sub.2 (.theta.) is zero for .vertline..theta..vertline.&gt;.theta.o and the ideal would be a rectangular diagram F.sub.1. With such a diagram, the spacing Do between sources could be equal to 1/sin .theta.. But to obtain this diagram, it would be necessary to have an antenna with infinite directivity requiring an elementary source of infinite spread.
These considerations are known and on pages 256-258 of the work "Phased array antennas" by Olmer and Knittel edited by ARTECH HOUSE, a solution to the problem is proposed, consisting in creating sub-arrays, i.e. in grouping together a number of elementary antennae and in feeding them appropriately from an energy distributor, so that these sub-arrays each radiate an approximately rectangular lobe with phase centers separated from each other by a distance such that the secondary maxima of the assembly of sub-arrays are shifted outside the principal lobe.
One embodiment giving a solution to the problem of the limitation of the number of phase-shifters with respect to the elementary radiating sources for obtaining a sweep range of the beam which is not too limited, may be found in U.S. Pat. No. 4,228,436 entitled "Limited sweep phase array". In this patent, there is essentially considered an interconnection circuit having T outputs and P inputs, T corresponding to the number of elementary sources contemplated and P to that of the phase-shifters. In this case, a number of circuits M is considered such that M=T/P.gtoreq.3.
One embodiment described in this American patent gives a good result with a sub-array comprising T.sup.2 antennae for an interval between sub-arrays equal to TDo, Do being the spacing between two elementary antennae. However, in this solution, since T is at maximum equal to 2 or 3, the sweep range still appears too limited for most applications.
Moreover, with the device described in this American patent an optimum amplitude and phase distribution over the different antennae, giving a rectangular radiation lobe, cannot be obtained, the advantage of the system is thus reduced.